Fertilizing fruit trees in pots is all together different than normal. But don't fertilize either until growth is well underway. You will need to fertilize much more often in pots because of smaller soil volume and the need to leach salts. Each time I water some water runs out the pot taking soluable nutrients like nitrogen out with the water. I fertilize about once a month in pots, just a small amount each time.

For your blueberries you need to use Ammonium sulfate or the acid lovers fert you find in the store. No nitrate nitrogen. For other fruit trees I mostly just use ammonium sulfate as well. But a little balanced fert like 10-5-10 would do as well.